1. Field of the Invention
The present invention relates to: a hydroxygallium phthalocyanine composite pigment effective as a photosensitive material for electrophotography, a material for photoelectric conversion elements, and a material for organic semiconductor elements; a highly stable electrophotographic photoconductor which is highly sensitive, can be used repeatedly, and stably output images under any environmental conditions; and an image forming device and a process cartridge for an image forming device, both using such electrophotographic photoconductor.
2. Description of the Related Art
There has been increasing demands for commercial or industrial printing using laser printers or digital photocopiers of electrophotographic system. Therefore, electrophotographic laser printers or digital photocopiers are required to provide higher quality of prints and to have higher reliability under severe conditions during use.
Electrophotographic photoconductors used for image forming devices of such laser printers or digital photocopiers would contribute to improvements of quality of prints and reliability by stably exhibiting sufficient charging functions and photo-induced discharging functions.
Currently, electrophotographic photoconductors using organic photosensitive materials are widely used for the reasons of cost, productivity, and low environmental loads. The important constitutional substances of a photoconductor include a charge-generating material and a charge-transporting material, and the characteristics of these materials give large influence to the charging function and photo-induced discharging function of the resulting photoconductor. The charge-generating material, which is related to the present invention, is principally required to have sensitivity to the wavelength of the light signal used for exposure. Laser diodes or LEDs are currently mainly used as an exposing unit, and the light having long wavelengths, such as an emission wavelength of 650 nm to 780 nm, is commonly used. As the charge-generating material having photosensitivity in such wavelength range, phthalocyanine pigments are especially widely applied.
Various phthalocyanine pigments are available for use, such as non-metal phthalocyanine pigments, copper phthalocyanine pigments, titanylphthalocyanine pigments, chlorogallium phthalocyanine pigments, and hydroxygallium phthalocyanine pigments. Among them, hydroxygallium phthalocyanine pigments are known to be stably used with a small environmental dependency regarding the photosensitivity thereof. Examples of hydroxygallium phthalocyanine are disclosed in Japanese Patent (JP-B) No. 3166293.
In the case where the phthalocyanine pigment is used for a charge-generating layer of an electrophotographic photoconductor, however, unintentional electrostatic latent image is formed due to accumulation of electrons, which may lower quality of prints. Hydroxygallium phthalocyanine is not exceptional, and the same problem is seen also when hydroxygallium phthalocyanine is used.
To solve such problem, the technique in which an electron-accepting material is added to a charge-generating layer has been disclosed. For example, there have been reported (see Japanese Patent Application Laid-Open (JP-A) Nos. 2006-018267 and 2005-208618) that, in the case where gallium phthalocyanine is used as a charge-generating material, a charge-generating layer and an undercoat layer both contain an electron-accepting material in the state of molecular dispersion, by dissolving the electron-accepting material in a charge-generating layer coating liquid, and applying such coating liquid. In this case, however, the contactability between the charge-generating material and the electron-accepting material may be insufficient because of poor solubility of the electron-accepting material or concentration deviation generated in the layer, so that generated electrons cannot be efficiently passed to the electron-accepting material. Therefore, the sensitivity is not improved sufficiently. Moreover, there have been reported (see JP-A Nos. 2008-015532 and 2007-034210) that an electron-accepting material such as polycyclic quinine pigment is added to a charge-generating layer using a phthalocyanine-based pigment as a charge-generating material. These proposals aim to improve sensitivity and to suppress residual potential with the charge-generating layer having such components. However, as the electron-accepting material is added to the charge-generating layer coating liquid to form the charge-generating layer, the location of the electron-accepting material present in the charge-generating layer is unintentional. Therefore, there are cases where the electron-accepting material is not sufficiently in contact with the charge-generating material within the charge-generating layer, and thus significant improvement of sensitivity cannot be expected. In addition, when a large number of prints are formed, a sufficient effect for suppressing the residual potential cannot be expected.
Moreover, a technique for mixing two or more pigments has been disclosed. For the purpose of expanding the correspondable exposure wavelength range, increasing sensitivity, or improving potential stability, for example, mixing two or more pigments has been proposed, such as a mixture of non-metal phthalocyanine and a fluorenone-based azo pigment (see JP-A No. 2001-290296), a mixture of a phthalocyanine compound and an azo pigment (see JP-B No. 3758246), a mixture of metal phthalocyanine and a perylene-hybrid pigment (see JP-B No. 3994638), and a mixture of a quinacridon pigment and a titanyl phthalocyanine pigment (JP-A No. 2007-334099).
Especially, regarding gallium phthalocyanine, there have been proposed techniques (see JP-B Nos. 4194184, 3880225, and 3792909, and JP-A No. 07-128888) in which an azo pigment and a gallium phthalocyanine pigment are used in the mixture for the purpose of expanding the correspondable exposure wavelength range and increasing photosensitivity. In any of these techniques, however, the pigments are mixed while dispersing the pigments, or mixed by mixing dispersion liquids each containing a pigment, and thus the charge-generating layer formed in such manner contains the mixed pigments having a distance to each other in the molecular level. Therefore, although desirable characteristics of each pigment can be provided to the resulting photoconductor, the synergistic effect between these pigments cannot be brought out, and thus any significant effect cannot be attained. Moreover, there has been proposed (see JP-A No. 2006-072304) that a phthalocyanine pigment and an electron-accepting material are formed into a composite, by presenting the electron-accepting material aside during the process of crystal conversion of phthalocyanine. However, many of the electron-accepting materials for use are generally poor in solubility. In the case where the solubility of the electron-accepting material is poor, it is difficult to make the charge-generating material sufficiently in contact with the electron-accepting material at the molecular level even though the electron-accepting material is allowed to be present aside of the phthalocyanine during the process of crystal conversion. Even if the electron-accepting material is adjacent to the surface of the pigment, a sufficient effect cannot be attained. If the electron-accepting material having excellent solubility is used, even though it is formed into a composite, the electron-accepting material is dissolved out at the time when a charge-generating layer coating liquid is prepared by dispersing the formed composite in an organic solvent. Therefore, desirable effects cannot be attained.
Moreover, there has been proposed a technique in which a pigment of a charge-generating material is used in the form of a mixed crystal. For example, it has been proposed that a mixed crystal of gallium phthalocyanine and other phthalocyanine is used (see JP-A Nos. 09-143386, and 2006-299269). As a result of this, the dispersibility thereof is improved. However, decrease in the crystallinity thereof cannot be avoided because different molecules are combined to form a crystal, and thus sufficient sensitivity cannot be obtained.
Furthermore, there have been proposed techniques (JP-B Nos. 3838385, and 3635786) in which the electron trap is inhibited by introducing an electron-absorbing group into a phthalocyanine molecule. However, in this method, as there is a structural change in the phthalocyanine ring, desirable crystal shapes may not be attained, crystallinity thereof is decreased, etc., and therefore, the sufficient sensitivity thereof is not necessarily attained.
As mentioned above, it is desired to provide a charge-generating material in use for electrophotographic photoconductor, which is highly sensitive in a wide wavelength range, and stably exhibiting a characteristic of electrophotography without accumulating electric charge, and various developments have been attempt to this end. However, it is a current situation that there have not been realized a charge-generating material and an electrophotographic photoconductor comprehensively satisfying such characteristics.